AD is the prototypic cortical dementia characterized by memory deficit together with dysphasia (language disorder in which there is an impairment of speech and of comprehension of speech), dyspraxia (disability to coordinate and perform certain purposeful movements and gestures in the absence of motor or sensory impairments) and agnosia (ability to recognize objects, persons, sounds, shapes, or smells) attributable to involvement of the cortical association areas. Special symptoms such as spastic paraparesis (weakness affecting the lower extremities) can also be involved (1-4).
Incidence of Alzheimer's disease increases dramatically with the age. AD is at present the most common cause of dementia. It is clinically characterized by a global decline of cognitive function that progresses slowly and leaves end-stage patients bound to bed, incontinent and dependent on custodial care. Death occurs, on average, 9 years after diagnosis (5).
The incidence rate of AD increases dramatically with age. United Nations population projections estimate that the number of people older than 80 years will approach 370 million by the year 2050. Currently, it is estimated that 50% of people older than age 85 are afflicted with AD. Therefore, more than 100 million people worldwide will suffer from dementia in 50 years. The vast number of people requiring constant care and other services will severely affect medical, monetary and human resources (6).
Memory impairment is the early feature of the disease and involves episodic memory (memory for day-to-day events). Semantic memory (memory for verbal and visual meaning) is involved later in the disease. By contrast, working memory (short-term memory involving structures and processes used for temporarily storing and manipulating information) and procedural memory (unconscious memory that is long-term memory of skills and procedure) are preserved until late. As the disease progresses, the additional features of language impairment, visual perceptual and spatial deficits, agnosias and apraxias emerge.
The classic picture of Alzheimer's disease is sufficiently characteristic to allow identification in approximately 80% of cases (7). Nevertheless, clinical heterogeneity does occur and not only is this important for clinical management but provides further implication of specific medication treatments for functionally different forms (8).
The pathological hallmark of AD includes amyloid plaques containing beta-amyloid (Abeta), neurofibrillary tangles (NFT) containing Tau and neuronal and synaptic dysfunction and loss (9-11). For the last decade, two major hypotheses on the cause of AD have been proposed: the “amyloid cascade hypothesis”, which states that the neurodegenerative process is a series of events triggered by the abnormal processing of the Amyloid Precursor Protein (APP) (12), and the “neuronal cytoskeletal degeneration hypothesis” (13), which proposes that cytoskeletal changes are the triggering events. The most widely accepted theory explaining AD progression remains the amyloid cascade hypothesis (14-16) and AD researchers have mainly focused on determining the mechanisms underlying the toxicity associated with Abeta proteins. On the contrary, Tau protein has received much less attention from the pharmaceutical industry than amyloid, because of both fundamental and practical concerns. Moreover, synaptic density change is the pathological lesion that better correlates with cognitive impairment than the two others. Studies have revealed that the amyloid pathology appears to progress in a neurotransmitter-specific manner where the cholinergic terminals appear most vulnerable, followed by the glutamatergic terminals and finally by the GABAergic terminals (11).